Here at the Lab we have access to a full kitchen, so I was able to easily execute this prank on-site. If your office doesn't have a stove (or another way to heat up water) and a refrigerator, it might be best to make the jello at home and bring it in.
You will need:
Lots of Jello. I'm talking at least a dozen boxes; I ended up using 16. Trust me. After 4 attempts at this (for you, folks) I can say that you will need an absurd amount of Jello. We're up-ing the density here, so keep in mind that depending on the flavor you choose it might come out a bit opaque. Feel free to experiment by mixing in some "clear" Knox gelatin, or playing around with food coloring.
Bowl, tupperware, bunt cake pan, etc. to put the Jello in - Make sure it's deep enough for your object.
Stapler, or whatever other object you want to suspend
Note: as is true with all pranks, try and skirt that line between funny and mean. Don't put anything in the Jello that could be damaged, or that the recipient would be seriously upset about. This includes electronics, important papers, sentimental items...you get the idea.
There are 2 approaches to suspending your object in the center of the Jello. First, you can fill your mold up halfway, allow that Jello to cool, place in your object, and then cover with another round of Jello to cool again. This method takes time, and an amount of patience that I don't have.
I opted for the 1-Jello-step process, and created a kind of hammock for my stapler, so I only needed to make the Jello once. For this method, you should also have:
Duct tape (lots)